If there are question marks in the pipe network longitudinal section (profile view) instead of station values, you need to set the reference alignment:
This exercise is not difficult for networks with one or two branches.
If you have a network with an increased amount of branches, this exercise might become quite tedious if you do it the standard Civil 3D way.
Devotech iDAS has a command to assign reference alignments for the entire network automatically. This command finds the closest alignment for each pipe and structure and assigns it as a reference alignment.